There are performance that take balls. There are performances that take the kind of stones that make Hillary Clinton's vote-counting look like amateur hour. And then there's the role of Kirk Lazarus in Tropic Thunder.

On its face, the idea is ghastly. Robert Downey Jr. stars as an Oscar winner who plays an African-American soldier in a war epic. And if you're thinking there's no way anyone would be dumb enough to play blackface for laughs in 2008, well, that's exactly what he does. The whole thing is a high-wire act over a pile of chain saws but Downey blows co-star Jack Black and Ben Stiller clear off the screen with his unblinking commitment to the gag. As Lazarus becomes more and more convinced he actually is black - to the alarm of his colleagues - the unhinged joke becomes clear. It's about race. It's about spoofing puffed-up Hollywood vanity. But more than anything else, it's about that essential element of all great comedy: danger. Somewhere, Lenny Bruce is smiling.